Conventional plate-and-frame assemblies, including humidity exchangers, are modeled after other well known devices such as heat exchangers and fuel cells which also use a plate-and-frame design. In conventional plate-and-frame assemblies a plurality of rigid plates are aligned one on top of another to form a stack of plates. The rigid plates can be made from different materials, depending upon the application and factors such as the operating environment, and weight, size and cost constraints. For some applications, the rigid plates are made from metal whereas in other applications the plates are made from rigid plastic or composites comprising resins and fibers. For example, conventional plate-and-frame heat exchangers typically use rigid metal plates to fluidly isolate two different fluids, but heat is conducted through the rigid metal plates to transfer heat from one fluid to another fluid. Heat exchangers which use metal plates typically use gasket seals made from soft deformable materials such as cork or rubber to provide fluid tight seals between the plates.
Plate-and-frame fluid exchanging assemblies are different from conventional heat exchangers. For example, plate-and-frame fluid exchanging assemblies may be used for exchanging water vapor between two fluid streams. A barrier which separates the two fluid streams must be selectively permeable to water vapor. Plate-and-frame humidity exchangers commonly use substantially impermeable rigid plates with open-faced fluid channels formed therein and a membrane interposed between the plates. The membrane is selectively permeable to water vapor so that water vapor in a humid fluid stream may be transferred through the membrane to a less humid fluid stream. Because the membrane is selectively permeable it serves as a separator which prevents fluid components other than water from being transferred between the two fluids.
To increase the capacity of a plate-and-frame humidity exchanger the useful surface area of the membrane may be increased. One method of increasing the membrane surface area is to use a plurality of rigid plates and membranes stacked one on top of the other for increased membrane surface area. In a stack arrangement, it is well known to provide each plate with openings which align with openings in adjacent plates to form internal fluid manifolds for distributing fluids to and from fluid channels associated with each of the plates in a stack. With conventional sealed plate-and-frame assemblies compliant gaskets are used around manifold openings and perimeter edges of each rigid plate to prevent fluid leakage and to also prevent different fluid streams from mixing with each other within the stack. The gaskets are conventionally made from materials which are softer than the rigid plate materials and the membrane. Gasket materials are substantially impermeable to the fluids which are to be flowing through the plate-and-frame assembly. Unlike rigid plate materials which are structural elements, conventional gasket materials do not contribute to the structural framework of humidity exchangers. Accordingly, conventional gaskets used for humidity exchangers may be made from deformable materials such as, for example, elastomers.
The gaskets may be pre-formed by known methods and placed between the rigid plates and membranes during assembly of the stack. The rigid plates are typically formed with a depression for receiving the gasket and holding it in position. A problem with gaskets is that it is labor intensive to position the gaskets between each rigid plate and membrane during assembly. There may also be leakage problems with a gasket seal if the gasket is misaligned. For example, gasket misalignment could be caused by applying the compressive force to the stack unevenly.
An application for plate-and-frame humidity exchangers is for use in conjunction with solid polymer fuel cells, namely for transferring water vapor from the oxidant exhaust stream to at least one of the reactant supply streams. Solid polymer fuel cells employ an ion exchange membrane such as a proton exchange membrane as the electrolyte. The water content of the fuel cell membrane typically affects the ion conductivity of the fuel cell membrane. The proton conductivity of a fuel cell membrane generally increases as the water content or hydration of the fuel cell membrane increases, therefore it is desirable to maintain a sufficiently high level of hydration in the fuel cell membrane. If the fuel cell membrane becomes dehydrated, the reduction in ion conductivity may result in cell reversal and the generation of heat, both of which may cause permanent damage to the fuel cell components. Accordingly, it is necessary to manage the moisture content of the fuel cell membrane to prevent damage to the fuel cell components and to maintain the performance level of the fuel cell.
The ability of reactant gases to absorb water vapor varies significantly with changes in temperature and pressure. Therefore, it is preferred to humidify the gaseous reactant supply streams at or as near as possible to the operating temperature and pressure in the fuel cell. If the reactant gas is humidified at a temperature higher than the fuel cell operating temperature this can result in condensation of liquid water occurring when the humidified reactant gas enters the fuel cell. Condensation may cause flooding in the electrodes which can detrimentally affect fuel cell performance. Conversely, if the reactant gas stream is humidified at a temperature lower than the fuel cell operating temperature, the reduced water vapor content in the reactant gas stream could result in membrane dehydration and damage to the membrane. Thus, the reactant streams are often heated prior to introduction into the fuel cell.
The reactant streams exiting the fuel cell or fuel cell stack typically contain product water, as well as water vapor which was present in the humidified stream delivered to the fuel cell. In particular, as the oxidant stream travels through a fuel cell, it absorbs water that is produced as the product of the electrochemical reaction at the cathode.
Accordingly, because of the benefits associated with humidifying the reactant supply streams and the water present in the exhaust streams, it is desirable to recover the water present in at least one of the exhaust streams to humidify at least one of the reactant supply streams. For example, it is desirable to provide an improved humidity exchanger for transferring water vapor from the oxidant exhaust stream to the oxidant supply stream, wherein such improvements contribute to ease of assembly and reductions in the cost of production compared to conventional plate-and-frame humidity exchangers which use rigid plates and gasket seals.